Field of the Invention
The present invention relates generally to a tape automated bonding (TAB) process of attaching leads to a semiconductor die, and more particularly to a method of attaching tape leads which extend across the interior surface of the die rather than radially outwardly from the die, thus enabling lead placement on opposite sides of the die in interdigitated fashion allowing die to be installed on a circuit board more closely adjacent than has previously been possible.
Assembly technology used in the manufacture of integrated circuit chips has been evolving rapidly, moving away from traditional packaging as individual pin-mounted devices which are installed on printed circuit boards (PCB's). The maximum density of such pin-mounted devices is limited by the physical size of the package the die is mounted in, a constraint which the increasing complexity of circuits, particularly in memory applications, has found to be unacceptable.
The alternative has been found in chip-on-board (COB) technology, in which the semiconductor die is physically mounted on a printed circuit board. In chip-on-board construction, the chip is adhesively secured in position on usually the surface of a printed circuit board. Typically, in chip-on-board applications, gold wire bonding has then been used to connect terminal pads on the die to lands (bond pads) on the printed circuit board. This construction has increased packaging density, but presents several disadvantages.
First, wire bonding has distinct density limits, beyond which reliable construction is simply not physically possible. Since the wires extend upward at one end from the terminal pads on the die, and upward at the other end from the lands on the printed circuit board, they are subject to problems with adjacent wires shorting together in closely spaced construction. In addition, the height of the assembly is significantly increased by the construction, since the wires lead upward at their connections to the terminal pads on the die. The lead length and conductor shape in wire bonding technology are not optimized, resulting in large lead inductances and lead-to-lead capacitances.
Lead technology took a quantum leap forward with the development of tape automated bonding techniques. In tape automated bonding, a pattern of flat leads are created on a segment of tape. An inner-lead bonding (ILB) operation is carried out to connect the flat leads to the terminal pads on the die. The die and its attached leads are removed from the tape, and the leads are formed into a gull wing configuration. The die is then adhesively mounted onto a printed circuit board, and an outer-lead bonding (OLB) operation is carried out to connect the flat connectors to the lands on the printed circuit board.
For an excellent introduction to tape automated bonding, see "An Introduction to Tape Automated Bonding & Fine Pitch Technology," technical report SMC-TR-001, published by the Surface Mount Council in January, 1989. This technical report is hereby incorporated herein by reference.
The tape used in tape automated bonding may be single-layer tape, two-layer tape, or three-layer tape. Single-layer tape is a conductive foil without any insulating layer. Two-layer tape is a conductive layer adjacent an insulating layer. Three-layer tape uses an adhesive layer between a conductive layer and an insulating layer. Additional special application tapes may be used with multiple metal layers.
The metal layer is made into a pattern of leads using photo imaging and etching processes well known in the art. The inner-lead bonding requires a procedure known as "bumping," which is the formation of a raised metal feature on either the die terminal pads or on the ends of the flat leads on the tape to be attached to the terminal pads on the die. The tape is positioned with respect to the die, and a sequential inner-lead bonding operation may be performed using either thermocompression, thermosonic bonding, ultrasonic bonding, laser bonding, or reflow bonding. Alternately, simultaneous inner-lead bonding may be performed using either thermocompression bonding or reflow bonding. At this point the die may be tested using the newly attached leads.
At this point the die together with the leads and the portions of the insulating layer adjacent the die (hereinafter called the leaded die) may be excised from the tape. The leads may then be formed in gull wing fashion if desired, extending down from the top of the die. At this point, the unattached ends of the leads are also formed for attachment into a flat array. If desired, the die may be encapsulated with a protective coating to protect it against contaminants and the environment.
The leaded die may then be installed on a printed circuit board. Typically, an adhesive is used to retain the die in place on the printed circuit board, and to provide heat sinking between the die and the printed circuit board. An outer-lead bonding operation is then performed to attach the flat leads to the lands. This bonding operation may be performed a lead at a time using a single-point bonder and thermocompression bonding, thermosonic bonding, ultrasonic bonding, reflow, or the use of conductive adhesives. Alternately, simultaneous bonding of all of the leads to the lands may be performed using thermocompression bonding, reflow bonding, or the use of conductive adhesives.
The use of chip-on-board technology and tape automated bonding technology together has thus allowed a quantum leap in miniaturization efforts, both in board area and in component height due to the gull wing lead configuration. The problems associated with wire bonding are eliminated, with lead density increasing dramatically and shorting being eliminated as a problem. Lead inductances are lowered, and lead-to-lead capacitance is minimized. Additional advantages are stronger lead connections, the ability to pretest and burn in the chip before installation on the printed circuit board, the requirement of less gold than wire bonding and the ability to mass bond, both of which lower costs, and the enhancement of high speed operation due to minimization of lead length.
Thus, tape automated bonding as used to implement chip-on-board construction has resulted in a package which has been thought to absolutely maximize packaging density, minimizing the board space needed to implement a particular design. The area required to install a chip and its leads is the actual area of the chip, plus the area required by the leads surrounding the chip. In chips having a large number of leads, the leads radiate outwardly from all sides of the chip. In chips having a smaller number of leads, the leads may emanate from two opposite sides of the chip.
One area in which is has become increasingly more important to concentrate a large number of chips in the smallest area possible is in the memory expansion board area. Personal computers increasingly have had applications in which it is desirable to have a large amount of memory. The race to put the most memory into the least space has been incredibly competitive, and has used the chip-on-board and tape automated bonding technologies described above.
It is accordingly the primary objective of the present invention that the chip-on-board and tape automated bonding technologies be utilized to further minimize the board area required to mount chips such as, for example, memory chips. It is thus an objective of the present invention to reduce to an absolute minimum the board space required to mount chips, thereby maximizing density to the greatest degree possible. In increasing the density of chip mounting, none of the advantages obtained through tape automated bonding may be sacrificed to any degree. The cost of the increased density must be minimal to enable that more compact designs may be sold at a price not significantly higher than competing lower density designs. It is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.